Tieflings are descendants of evil outsiders. Their tainted blood grants them heightened reflexes and cunning, but also tends to disturb people with whom they interact.

Because they are descended from evil outsiders, those who know of their ancestry immediately consider most tieflings evil and untrustworthy. Not all tieflings are evil or untrustworthy, but enough are that the prejudice tends to cling. Some tieflings have a minor physical trait suggesting their heritage, such as pointed teeth, red eyes, small horns, the odor of brimstone, cloven feet, or just and unnatural aura of wrongness. Those descended from an infernal minion of a deity often carry a birthmark of the deity's unholy symbol or another trait related to that faith.

[Source: Forgotten Realms Campaign Setting by Greenwood, Reynolds, Williams, and Heinsoo, published by Wizards of the Coast, page 20.]